operations: fix setting the timestamp on Windows for multithread copy

Before this fix we attempted to set the modification time on the file
when it was open. This works fine on Linux but not on Windows. The
test was also incorrect testing the source file rather than the
destination file.

This closes the file before setting the modification time and fixes
the tests.

Fixes #3994

fs/operations/multithread_test.go

@@ -138,7 +138,7 @@ func TestMultithreadCopy(t *testing.T) {
 			assert.Equal(t, src.Size(), dst.Size())
 			assert.Equal(t, "file1", dst.Remote())
 
-			fstest.CheckListingWithPrecision(t, r.Fremote, []fstest.Item{file1}, nil, fs.ModTimeNotSupported)
+			fstest.CheckItems(t, r.Flocal, file1)
 			require.NoError(t, dst.Remove(context.Background()))
 		})
 	}